ELECTRICAL RESISTIVITY CHANGES
AND THERMAL ARRESTS
ACCOMPANYING THE AGE
HARDENING OF
ALUMINUM ALLOYS

Thesis For The Degree of M. S.
FRED L. REYNOLDS
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Electrical Resistivity Changes
and
Thermal Arrests
Accompanging the Age Lardening

of

gluminum Alloys

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Y7." J? . Ii ‘. r- ”. ‘ 4' ‘I , r", ‘q . l" 'r“ T’\ A ' f“ ‘ ‘r‘ 4" 4‘ ““

Lds lirst lllbu on the -eculidr i,z'o-gerties tnut s0ne of

the light metal alloys disgley “Z“fl tcc.eul heat treated.

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V . l ,0
Sgt/‘33, CL

"‘03. pert}; “LI-net some of the aluminum {1103’s

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- r ‘V‘l V‘ ‘ ”‘1 . r«I .1132. 1 1 A). ". r! ' g N -’)r- fl“1r'\'»'1 ' 1 ' ‘.. ‘Y 'J'
ieideninb Qitui UClLQ LuehChpd i A gust celou the

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l

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eutectic point, is well known. The ;l iORuAOL res :iret

observed hv Tiln of Germany 121 his tort in 1205-11.

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Ta ")““‘1' 1 13A

~ I 1" "~“ 1‘, '1" . ".’. I': ‘3""“‘~~‘ ,. " f .1 o; ‘1 '
everul genre this Les unCAElainee, end it lendined

(O

For
so until the work of E. D. ierice end his co-“orlers in
13191. At that time, the idea of the heyi ing effect of
the herd CuAln ps rticles in the ”loun-mfss of aluminum res
advanced. Zhis conclusion was based uyon the thermal arrests
that were ocsc rVed on reheating a cuenched uneged specimmi of
durelumin. 7t was believed that these arrests were cvid3.ces

.L.

of a 1recicitation of the i “rt cles of CuAlg of e very highly

dispe ersed nature and of colloidal size. furthermore, tie

maximum l rdr mess that could be produced \;es dcpe dent on 31

no.

average critical sizeof the precipitated particles. Later,
4..

(198 -25) Jensen and Curler, working at the fictional -hycicel

~

laboratory, submitted the theory that ngbi was a much more

'“ v:

inqortent influence in the ego hard;hln: than CuAlQ“ L “-

<3ver, we have alloys such as "25s" thick are iree from Kgi;i

61nd hardened by CuAlg alone. Conversely, alloys such as ”file"

~"'~ ”—N 5". r r" “ 7-““"'-"1 u . 1‘ r~r
conta_n hgzoi alone as the hu‘dCMlLS constituent, u.e con-

k.)
siderablv at room temperature. It is notcvorthy that th:

former does not a e appreciably at room temperature, but

N
x.)

reguires ele'ated temperatures to bring about this efiect.
”he theories advanced by these tier}:e s, links well

with the slip interference theory adtenced by Teffries and
hrcher (1921)3. Although the proof submitted in favor of
the precipitation tieory is largely of an indirect nature,
it has been generally accepted by present day metallurgists.
Perhaps the main objection to the theory of precipitation
as applied to the age hardening of duralumin is the appareit
anomaly of electrical resistance changes during age harden-
ing+.

When a specimen is given the solution treatment 1.9.
quenched from just below the eutectic temperature¥ the re-
sistance shows a decided increase which conforms to the
idea given below*. However, on ageing the alloy its resist-
ivity continues to increase slightly.

With these facts in mind, the following work was under-
'taken to discover, if possible, this apparent exception u: an

otherwise plausible theory.

+ It has been considered that in general the resistivity of
an alloy is increased when an aggregate is changed to a solid
solution, and conversely that the resistivity is decreased
when a solid solution decomposes into an aggregate.

The author wishes to eXpress his indebtedness to
3. H. Dix, Jr., of the Aluminum Company of America for
the suggestion of the problem, and to H 3. Publow,
Instructor and friend, under whose direction this work
has been carried out.

Kany thanks are due to R. 5. Archer of the nluminum

Company of America for his kindly advice and assistance.

June 1929. jM/W

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Then the Work was Iirst started in duralumin,

3.!

there VLs 110 fabricated stock of knom1 comgositi on at

hand. AccorCin3ly, the several metals re ui red Iere

"

collected together and sixty (60) test bars mere cast

5

and machined for the study. The casting and study of

11

the cats alloys constituted the first orpcriment of time
York.

L ter, by suggestion of u. H. Dix, Jr., the invest-
igation of electrical resistivity of rolled duralumin on
a3ein3 was uniertalen. The 175 petal selected va Lindly
supplied by the nluminun Loréuiy of unenica. liis in-
vestigation of the lec trical resistivity changes durin3
the process of a3 e ruruLnin; after he solution tr atncnt,

‘

and the accomyany'n3 Iardness, constituted the second
GZLf 311101113.

She Lork of T. D. Lorica and his associates on the
thermal arrests in the in atir 3 of unsged sgecimcns, indicated

that there was room for further investigation along tLLs line.
Since dura lumin is rather complex in its constiju ion, it was
considered that this study'ni3ht b-tt r us ca: ricd cit with
er aluminum alloy. ¥Lis study of the thermal

critical points in a Lure cepger—aluminun alloy made up the

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Lercen .a3 e of cop.er1n tn melt. in this Connection, Lhe

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loric iu:1ribbon was aided eirLctly b0 thy MJlt be11o1e
casting. were was taken to he 9 the 1.213 sium under the
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adiitions to the melts. Lhis alloy seemed

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to be much tougher and harder than pure aluminum,
but not as hard as the copper alloy.

Powdered silicon was added directly to the melt
before casting. The melt was superheated about 10%
before these additions were made.

The test bars were sand cast on end, four in a
mold from a Sprue in the center. After several unsuccess-
ful attempts, it was found that the gates must be out
very heavy. Another item to which little attention was
paid at the time of casting, is the casting temperature.
This should be just above the melting point. Otherwise,
the metal is very porous. Many of the fractures of the
bars obtained shov porosity, shrink holes, brown oxide,
or chalky surfaces. Hence, the physical tests as given
here are inferior to the intrinsic qualities of the metals.
Many of the test bars broke in the head, due to shrink-
age in the t0p end of the cast bar.

The bars were machined to the standard test size and
soaked in a furnace at 9450F for three hours. After that
time they were quenched in water and started to age. Half
of them aged at 105°C and the other half at room temperature.
As the ageing progressed, tensile tests were made. The re-

sults of these tests are given in Tables II, III and IV.

Table II

 

 

 

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better on th_s slloy then the Oh are, better results are Obtilfiiu
ntv' the ageine takes place a 10590. since no gsrticular *sye
Wes tegex n the casiixs of phis :lloy, it seems to be Luck ug‘c

deggndxble the tge others.

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is youred at tle correct tcmgeratute, uow d CLS+iL‘E Lill

result, especially in the Kfl—Li—ll ollov

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this pnfalldcnt dealt with the algCiIlC§l TCSle-

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l“"‘""“-'ie‘:ium R'
bk 1A.. ‘. A . U’.)
c ~ g,
Langaiese .0”
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silicon ,Uufl
Iron ,3”
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alurziinun 9»; ,oolr,

‘—~ 17 ‘ ' ‘41 ,- f, an
(u,‘<liiierunice)

The structure of the annealed metal is shown in 11;. I
and la. THO strips, 3/8” x 14” were cut from a sh;et of
rolled stock .035” thick. the apparatus for nakin; these
measuremants is shown in Fig. II. A current Cf 6 - lO anps.,
supplied by a direct current generator, was passed thru hie
strip. Ihe drOp in potential across the strip was measired
by a potentiometer set-up as shown. The resistance of tha
strip was computed from these data, using Ohm's law.

after the solution treatment was given, one of the strips
aged at room temperature, while the other was aged in a dry-
ing oven at 105°C. The resistivity changes are tabulated in
Table V and exPressed graphically in Fig. III.

In conjunction with the resistance measurements, file

hardness was observed. The changes in hardness are tabukited

in Table V and plotted in Fig. V.

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FIGII

lable V
mime 2200 10500
J. .11
Aged Resistance Hardness fiesistance Iardnass

Annealed .001370 ohms 55 .001405 ohms 55

Quenched .001925 78 .001935 78
50 min .001960 .002038
1 Hr .001993 .002058

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an ah:.ealbd strio of 7 r'luM1a is 'iven the .:olution trcat-
me1t and subscguent y s3ud at IDOL tzw;sratuie.

I0 014 1'ca tlat UL; reriS‘ititv ”f an 'llov is in-
crease :Len an aggregate is changed into a solid solui-

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of a conductor is given by the iormul a:

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loixa, t1131r~sis1hnlcc Clnniges Inn,cu riist c»,e1ue 11) a CIlLLQe

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-,... .-- -- ,. , fl .. .' ,3 , --‘ .\ , .. -1. ,., a.“ ....,;1 ° .

ngui,cu“lb, dim :oliu solutions 01 tiese cenpo*lus 1n alum-
,- r. .. ,., ' ,‘ ,' ' .:. ‘n .1. "1" .i :3 ' i ' V.‘ .L,_.. . . | ,,-

1hun are .1uely Lillerunb, “e rzusi take each one into aucou t

‘ O " I J) go.) o .' f c o t u ‘ ' .‘\‘

JILGU or "elicctite specific res; ta.ce".

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basis of chah'ing ta ass 01 this aijective specific TLELSt-

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:n t-e lirst place, the resistinity oi sufJ aluminum

much letter than that of 6:113th the zine‘tall'ic co 11100301535 or

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of solid ’1uilOLo 01 these con Luminune. cepfldly,

the resistivity of CuAlg is less than KggSi+. the fact that
a greater increase in resistivity is observed on the strip
aged at room temperature, during which only Mggsi is pre-
cipitated seems proof enough for this.

Referring to Fig. IVa, there exists in the annealed
state large particles of intermetallic compounds surrounded
by a matrix of nearly pure aluminum. A cross section of
a bar cs such a material could be represented as shown in
the figure. Although much of the area is taken up by inter-
metallic compounds which have a comparatively high resist-
ance, the matrix is of nearly pure aluminum which.makes the
effective specific resistance quite low.

When the alloy is given the solution treatment, these
intermetallic compounds are held in a supersaturated sold: -
ion of a complex nature and a cross sectional view wzuld be
homogeneous as in Fig. I'V'b. Work on several metals6 show
that the resistance of a solid solution is higher than its
component metals. Very little, if any, work has been done
in comparison.of the resistivity of intermetallic compounds
and solid solutions, however. The facts presented in this

mark show that the resistivity of these compounds are greater

+ It has been shown that in general intermatallic compounds
made up of two elements which occur in the same or adjacent
groups of the periodic table (as magnesium.and silicon)
have a high electrical resistivity. Conversely, two elements
widely separated in the periodic-table (as Dapper and Aluminum)
produce a compound of low electrical resistivity.

 

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than their solid solutions. Since pure aluminum has a
lower conductivity than its solid solutions, the solution
treatment would be eXpected to markedly increase the resist-
ance. This is found to be true (fig. III).

For a short time at room temperature, the ageing pre-
cipitates HggSi from solution but Guile is retained largely.
Chis fact is evidenced by alloys such as "513" which harden
readily at room temperature and contain KgsSi alone as the

ardening agent. Such alloys as "'5s" tith no KggSi, but
containing Cunlu do not harden apnre01uoly at room trrm t-

«(:4-

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the quenched piece starts to age at room t mioreture me have
hgsSi comin" out oi solution (113 . 1V0) which, as explained

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above, has a high resistivity. The Cunls remains in solution

. -1 . 4“ -,-.— f~ - - . ‘Qr- " . ra - 4‘ . T t ‘5 N, n J-‘
1nd gives the Ldtrlh a Ligh reels.ivit1 al 0. hence, as the

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piece fges ior a sLort tine at noon b ‘ tu re, LuUrJ n5 a

further iii-.CI'LELSC in resisttuice. --s the («gt-.1113 co:1t1nuos, 1;:ore

Lg~3i :nu coho oi tie ounl~ cones ut o“ sol *ion. 3.3 has

a dual e33 ecc. Tirst, t1 composition of the zat‘ix :3 CJ n ed
so that its resistivity is lJ ed and secomi, CUnlz is gre-
cyipitatcd, riiich ha. :i lozeI1‘wmsistivit3rilini the ikm:n:r ;O;id
solution. Jherofore, ageing at room t uperaturo Lor about tro

days, the resistivity starts to deer use. niter this -rffcct
is 001plctod (in about 4 days) a ‘ort of an o uilibrium is

reached and no :irther change in resistance is noticed.

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Cipitate. “itcr the -irs l‘L ends in “-th in I ion a 1““-

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matrix approaCics were luiin‘m cue LO lretipitation, so that
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r ‘ -. .., 4-1-. 7 A ,_ ' I ,-~ . , -\ - ., \ .L ‘ .' a” ‘ .' 7.. ' .- ‘ -.~. . p .. :;
-hen bLe curve ceilinos slur l; ntil an e Ulllurlufl is ioach u

between the resistivity o; the Trcciwitated co 1pcrncs ant

solid 90

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Sunlg vhich SlUTO oszdly has a co1. a: ativcly lor resistivity.
4s a nutter of fact, then xhe solution treatment is gnrcn in
lloy of this ‘"1e,re do not lave com lets solution of the
constituents as shown in Tir. Vi. Lonevor, this excess is

the same in all conditions.of Changes ob-

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inclined t :ollov the T“SlSElTity curve.
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.‘~ c_\,__1_$ .... .... ..'.-‘_, n .... , -1 ‘_ ."
sesveu thernal a-rcsts on rch.at1-g iresbly ;u:nched

aluminug alloys. This was Very logically explainvd by

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prec1p1tat1.5 .rom the supersaturated bOlULlOn. “OR-

.1.

ever, they were not able to find any difference in the
microstructure of a piece before and after reheating
subsequent to the solution treatment. Lince the material
used for their work contained other elements in apprecnible
percentages as well as cop;er, it may be tiat their results
were influenced by the presence of these other elements.
With this in mind the third eXperiment of the series
was made to check the previous work and to supplement it
with a micrographic analysis.
A copper¥aluminum alloy was used which was very pure.
The percentage of coPper was about 5.5% with Less than
.05% or other impurities. The alloy was held at 1005°F
for 72 hours and quenched in water to get complete solution
(Fig. VIII). The thermal samples were carefully reheated
on a double sand bath. The temperature was taken at equal
intervals of time by a mercury thermometer inserted in
a hole drilled into the piece for the purpose (Fig. VII).
This alloy was studied with a Leeds Northrup critical
point recorder in an endeavor to determine the thermal
arrests. This was unsuccessful however, as the points are

masked in such a way that this instrument will not detect

them.

 

 

 

 

 

 

 

 

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Discussion of Results of sXperiment III

The amount of copper in the alloy used in this
experiment seems to be near the limit which is soluble
in aluminum at 100500. In fact, several authors have
given the limit somewhat lower than this. However, the
picture shows the complete solution has been made (Eng.
VIII). The inverse rate curve is plotted for the re-
heating and is given in Fig. IX. The Spacimen was
covered with water glass before reheating so that a
sharp arrest is made at the boiling point of water. A
slight break is noticed at 35OOF which is not accoum:ed
for except by inequalities in the heating rate. it
SSOOF a sharp break is encountered and another apjears at
6000?. This double arrest has been checked by running
another sample and is confirmed by it.

Figs. I, II, III, show the changes that take plice
when this alloy is reheated at 5400?, 57003 and 6000?
respectively. The firs temperature (béOOI) is just
below the iirst critical point and shons almost no aro-
cipitation of ounlb. The internediate temperature (37001)
is between the two thermal arrests an; sh‘vs .rccipitation
within th, crystals. The t-.::_da:;nc:,r of bugl-

at the grain boundries is not apparun‘ in this piece.

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1. Bureau of vtenieice, Science 13;;r 847:
P. D. :rica; E. 9. let.nber1 Ln. 1. boott.

2. The Let3 lurgy 01 Aluminum and lluminum alloys,

.1. I; . ¢1l-CLerg) n.
3. The Slip Interference Theory of the Tardening of

jetals. Z. Jeffries and R. e. Archer.-Chem. and

Ketallirgi cal Engineering, June 15, 192

4. 1he Hrrderirr of Ietals oy Dispersed Constituaits
lrecipitated i‘rom Solid Solutions. ii. 3. archer
Trans. American Society for Steel Treating ,Eept.,

1926.

5. Science of Retals, page 230. u. Jeifries and 3- S.
Archer.

6. Llectrica l Conductivity of Ketals and Alloys. C. A.
AsI‘auS .

7. Trans. Am. Inst. of Linin3 and Eetallurgical
slaineers. Vol. L"‘V (192 O)

8. a Treatise on Engs ical C113mistry Vol. II, page 494.

Jig S. .LHCL leI'.

9. nl-3tron.nih oretische Stuuien an meta llis c} n1 festei
Losungen. von -uaolf 3chenck. nnnalen der Thysik

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